Hard backing bar for adjustable wire wound systems



Dec. 31, 1963 w. w. ROBINSGN 3,115,470

HARD BACKING BAR FOR ADJUSTABLE WIRE wouun SYSTEMS Filed Aug. 11, I961 United States Patent HARD BACKING BAR FOR ADJUSTABLE WIRE WOUND SYSTEMS Wayne W. Robinson, Covina, Calii, assignor to Edclilf Instruments, Monrovia, Califi, a corporation of California Filed Aug. 11, 1%1, Ser. No. 130,804 8 Claims. (Cl. 338180) This invention is directed to improvements in adjustable wire wound systems and more particularly to an improved apparatus for supporting the resistance wire of a variable wire wound resistor or potentiometer against the pressure contact of its movable arm.

With the advent of compact electrical system designs it has become desirable to provide wire wound systems, such as variable resistors and potentiometers, which achieve a maximum amount of overall resistance in a minimum of volume. Such a miniaturized wire wound system design has been recently developed and is the subject matter of the U.S. Patent No. 2,892,171 which issued June 23, 1959.

As described, to provide an adjustable wire wound system design which achieves a maximum of resistance in a minimum of volume without sacrificing structural ruggedness or economy, the wire wound system includes a length of fine resistance wire tightly wound around a hollow core element and a movable contact arm positioned within the hollow core element for movement along a longitudinally extending slot in the outer wall of the core element to make movable contact with the wound resistance wire passing over the longitudinal slot.

In making contact with the wound resistance wire, however, the movable arm, which is spring loaded, exerts a force of approximately grams on the wound resistance wire passing over the longitudinal slot. The force exerted by the movable arm tends to both stretch the wound resistance wire and separate the windings around the core element. Stretching of the wound resistance wire changes the resistance characteristics of the system, while the separation of its windings tends to cause the movable arm to stick between individual windings.

To overcome these problems it has been the practice in the past to provide a backing strip for the wire wound system along the wound resistance wire passing over the longitudinal slot. The backing strip is composed of a conductive material, such as copper, to provide a return current path between one end of the wound resistance wire and an output terminal located at the other end of the system. The backing strip is coated with a non-conductive material to insulate the backing strip from the wound resistance wire.

In practice it is diificult to find an insulating coating which provides adequate insulation for the backing strip at high temperatures. This is due in part to the difference between the thermal expansion of copper and the insulating material utilized to coat the backing strip. In particular, insulating materials which expand more rapidly than the copper backing strip tend to soften at temperatures of the order of 100 to 150 C. As the insulating coating softens, the contact arm forces the resistance Wire into and through the coating to make electrical contact with the copper backing strip, thereby short circuiting the wire wound system, causing it to burn out. On the other hand, if the coating material has a thermal expansion which is less than that of copper, as the copper strip expands the coating cracks and splits, again allowing the movable contact arm to force the wound resistance wire into contact with the copper backing strip.

Another problem frequently encountered under previous design conditions is that the insulating layer between the coil and the conductive backing strip may cold flow between the coil turns and produce electrical noise.

In view of the above, the present invention provides means for supporting a wound resistance Wire in an adjustable wire wound system which is uneifected by increasing temperatures. To accomplish this the present invention, in a basic form, comprises a rigid backing bar composed entirely of a hard insulating material in combination with the wire wound system.

As briefly described above, the adjustable wire wound system includes a length of resistance wire tightly wound around a hollow core element and a movable contact arm positioned within the hollow core element for movement along a longitudinally extending slot in the outer wall of the core element. A movable contact arm makes pressure contact with the wound resistance wire passing over the longitudinal slot. The rigid backing bar of hard insulating material is positioned along the wound resistance wire passing over the longitudinal slot to counteract the pressure contact of the movable arm. The backing bar is affixed to and forced against the wound resistance wire. By so afiixing the backing bar to the wound resistance wire the backing bar provides a smooth yet rigid support for the wound resistance wire, thereby preventing the wound resistance wire from being stretched or the windings separated by the pressure contact of the movable arm.

Preferably the rigid backing bar is composed of an insulating material chosen from a group of materials consisting of Pyrex-type glass, ceramics, lavas, aluminas, glass, dyall-phthialate plastics (glass filled plastics) and hard anodized aluminum. These materials have the characteristic of being hard insulators which have high melting points and low coefficients of thermal expansion. Thus, as the temperature applied to a backing bar composed of one of the hard insulating materials increases, the surface dimensions as well as the hardness of the rigid backing bar remain substantially constant. Accordingly, positioning a bar composed of one of the hard insulating materials along the wound resistance wire passing over the longitudinal slot provides a rigid backing support for the wound resistance wire which is substantially unaffected by increasing temperature and which enables the complete elimination of the objectionable insulating layer heretofore employed between the coil and a conductive backing bar.

In particular, as the temperature increases the backing bar remains rigid and the pressure contact of the movable arm is unable to deform the windings passing over the longitudinal slot. Also, the hard insulating materials do not cold flow between the windings of the wire wound system due to the pressure contact of the movable arm.

In a preferred form, the backing bar has curved opposing faces and is slightly narrower than the longitudinal slot to allow the backing bar to fit into the slot. In particular, the lower face of the backing bar is curved and is positioned to contact the wound resistance wire along the longitudinal slot. The upper face of the bar is curved to substantially conform to the curvature of the core element and cooperates with the inner surface of a sleeve of non-conducting material to form a snug continuous fit for the sleeve around the wound resistance wire.

To form a snug fit between the sleeve and the wound resistance wire, the backing bar is forced into the longitudinal slot. This depresses the wound resistance wire passing over the longitudinal slot into the slot to follow the contour of the lower face of the backing bar. Due to the curvature of the lower face of the preferred form of the backing bar, the wound resistance wire makes a tangential electrical contact with the movable arm which a is maintained irrespective of lateral rocking movement of the movable arm within the longitudinal slot.

The backing bar in its preferred form also has a longitudinally extending groove in its curved upper face. The groove extends the length of the backing bar. In practice, when the backing bar is combined with a wire wound system of the type described in the aforementioned 'U.S. patent, a conductor is positioned in the longitudinally extending groove. The conductor is longer than the backing bar such that portions of the conductor extend beyond the ends of the backing bar. The portions of the conductor extending beyond the backing bar act as electrical terminals for the fine resistance wire which may be coupled (as by spot welding) to the condoctor to provide a return current path from one end of the wound resistance wire to an electrical terminal located at the other end of the wire wound system. Since the rigid hacking bar is composed entirely of a hard insulating material and since the conductor is positioned in a groove in the upper face of the backing bar the return current path is insulated from the wound resistance Wire and is unaffected by increasing temperatures of the wire wound system.

The above, as well as other features of the present invention, may be more clearly understood by reference to the following detailed description when considered with the drawings, in which:

FIG. 1 is a cross-sectional schematic representation of a wire wound system utilizing the backing bar of the present invention;

FIG. 2 is a cross-sectional representation along the cutting line 22 represented in FIG. 1;

FIG. 3 is an enlarged view of the section 33 of FIG 2; and

FIG. 4 is a perspective representation of a preferred form of the backing bar of the present invention.

As indicated briefly above, the present invention relates to the combination of a backing bar with an adjustable wire wound system such as disclosed in the US. Patent No. 2,892,171, issued June 23, 1959, to provide rigid backing support for a wound resistance wire. A cross-sectional representation of such an adjustable wire wound system including the rigid backing bar support arrangement of the present invention is illustrated in FIG. 1.

The adjustable wire wound system includes a tubular housing having a relatively large section 12 and an end section 14 of reduced diameter-the end section 14 being threaded externally for mounting purposes. A hollow core element 16 of insulating material, such as Bakelite plastic, is included within the housing 10. The core element 16 has end sections 18 and 20 of a diameter substantially equal to the inside diameter of the housing 10 to allow the core element 16 to fit snugly within the housing 10. The core element 16 also has a central section 22 of reduced diameter. Included in a wall of the central section 22 is a longitudinally extending slot 24.

Wrapped tightly around the central section 22 of the core element 16 is a small diameter resistance wire, represented at 26. The resistance wire 26 is tightly wound around the central section 22 over the longitudinal slot 24 in the form of a helix.

Positioned within the hollow core element 16 is a threaded shaft 28. The threaded shaft 28 is mounted into a cylindrical insulating head 30 which is provided with a notch 32 for receiving a screw driver by means of which the shaft 28 may be rotated. The opposite end of the shaft 28 bears against a thrust bearing 34 which is retained in the inner end of a plug 36. The shaft 28 is loaded against the bearing 34 by a coil spring 38 which is supported between the head 30 and the housing 10.

The bearing 34 is a conductive disk and is embedded in the inner end of a bore 49 in the plug 36. The hearing disk 34 is center bored to receive a Cable 42 which is soldered to the disk face, as represented. This provides an electrical connection between the cable 42 and the threaded shaft 28.

Mounted on the threaded shaft 28 within the hollow core element 16 is a spring loaded contact arm represented at 44. The contact arm 44 is supported on a sleeve 46 which is threaded directly on the shaft 28. The sleeve 46 supports a boss 48 to which the contact arm 44 is attached.

As represented in FIG. 2, the boss 48, in being supported by the sleeve 46, projects into and rides along the edges of the slot 24. Thus, the sleeve 46 is prevented from rotating within the core element 16, and any rotation of the threaded shaft 28 causes a linear displacement of the sleeve 46 along the shaft 28 thereby moving the contact arm 44 along the wound resistance wire 26 pass ing over the longitudinal slot 24.

As represented in FIG. 1, the spring loaded contact arm 44 exerts an outward force on the portion of the wound resistance wire 26 passing over the longitudinal slot 24. With the wound resistance wire unsupported, this force (which approximates 25 grams) tends to stretch the resistance wire and separate the windings. The stretching of the resistance wire 26 modifies the resistance charcteristics of the adjustable wire wound system, while the separation of the windings passing over the longitudinal slot 24 tends to cause the contact arm 44 to stick between individual windings. To overcome these problems, the present invention includes a hard, rigid backing bar 50 composed entirely of an insulating material mounted along the wound resistance wire 26 passing over the longitudinal slot 24. The backing bar 50 is coupled to the wound resistance wire 26 by an epoxy resin, represented at 52, to provide backing support for the portion of the wound resistance wire passing over the longitudinal slot 24, against the pressure contact of the arm 44.

Preferably, the backing bar 50 is composed of an insulating material chosen from the group of materials consisting of Pyrex-type glass, ceramics, lavas, aluminas, glass, dialyl-phthalate plastics (glass filled plastics) and hard anodized aluminum. These materials are hard insulating materials as opposed to soft insulators such as thermal plastics of Teflon, nylon, and Formva-r, and possess a high melting point and a low coefiicient of thermal expansion. Thus, mounting a backing bar composed of one of the above-mentioned hard rigid insulating materials along the outer surface of the wound resistance wire passing over the longitudinal slot 24 provides firm support against the pressure contact of the arm 44 and forms a smooth contacting surface for the arm 44 as it moves along the inner surface of the wound resistance wire 26. Further, due to the high melting point and low coefiicient of thermal expansion of these materials neither the surface dimensions or hardness of a backing bar composed of one of the hard insulating material will appreciably change with increasing temperatures. Accordingly, the rigid backing support provided by the hacking bar of the present invention to prevent any deformation of the windings in a wire wound system is maintained at temperatures well in excess of C.

As represented in cross-section in FIGS. 1, 2 and 3, backing bar 50 is slightly narrower than the longitudinal slot 24 to allow the backing bar 50 to fit down into the longitudinal slot 24 and is preferably cross-sectionally shaped in the form of an ellipse. Thus its upper face 54 and its lower face 56 form curved surfaces. The curved upper face 54 of the backing bar substantially conforms to the curvature of the core element 16 as well as with a sleeve of insulating material 53 which extends between the end sections 18 and 20 over the wound resistance wire 26 and the central section 22 of the core element 16. More particularly, the upper face 54 of the backing bar 50 cooperates with the inner surface of the sleeve 58 to provide a snug continuous fit between the inner surface of the sleeve 58 and the outer surface of the wound resistance wire 26.

To form the snug fit between the sleeve 58 and the wound resistance wire 26, the backing bar is forced down into the longitudinal slot 24. This depresses the portion of the wound resistance wire passing over the longitudinal slot down into the longitudinal slot. In being forced into the longitudinal slot, the wound resistance wire conforms to the lower surface 56 of the backing bar making a tangential contact with the arm 44 which is maintained irrespective of lateral rocking movement of the contact arm 44 as the threaded shaft 28 is initially turned within the hollow core element 16.

Included within the upper face 56 of the backing bar 50 is a longitudinally extending groove 60. The groove 60 extends the length of the backing bar 50. Positioned within the groove 69 is a conductor 62. The conductor 62 provides a return current path from one end of the wound resistance wire 26 to an external output terminal (not shown). Since the backing bar 50 is composed of an insulating material the conductor 62 is insulated from the wound resistance wire. The conductor 62, although flexible, is substantially rigid compared with the fine resistance wire 26 and is longer than the backing bar 5t). Accordingly, portions of the conductor 62, represented at 64 and 66 in FIGS. 1 and 4, extend beyond the ends of the backing bar 5b to act as terminal posts. As represented at 68 in FIG. 1, one end of the wound resistance wire 26 is coupled to the end portion 66 to provide an electrical connection between one end of the wound resistance wire 26 and the conductor 62. This may be accomplished by winding the wound resistance wire around and soldering it to the end portion 66 or, more preferably, by spot welding the end of wire 26 to the top of the end portion 66. In a like manner the end portion 64 is electrically coupled to a cable 70. Thus a return current path is provided from a cable 72 through the wound resistance wire 26 and the conductor 62 to the cable 70 providing for operation of the adjustable wire wound system, either as a rheostat or a potentiometer.

What is claimed is:

1. In an adjustable wire-wound system including a resistance wire wound over a hollow core element of insulating material, the core element having a longitudinally extending slot disposed in its outer wall, and a movable contact arm passing within the core element along the longitudinal slot, the contact arm making a movable pressure contact with the wound resistance wire passing over the longitudinal slot, means positioned on the wound resistance wire along the longitudinal slot remote from the movable contact arm to provide backing support for the wound resistance wire against the movable pressure contact of the movable arm comprising a hard, rigid backing bar composed of a hard insulating material chosen from the group of materials consisting of Pyrex-type glass, ceramics, lavas, aluminas, glass, dialyl-phthalate plastics and hard anodized aluminum, the bar being slightly narrower than the longitudinal slot to allow the backing bar to fit into the slot and having an outwardly curved lower face contacting the wound resistance wire along the longitudinal slot, and an upper face having a longitudinally extending groove for receiving a conductor, the upper face being outwardly curved to cooperate with means for forcing the bar into the slot to depress the wound resistance wire passing over the longitudinal slot into the slot to follow the contour of the curved lower face of the backing bar whereby the wound resistance wire makes tangential contact with the movable arm within the slot.

2. In an adjustable wire-wound system including a resistance wire wound over a hollow core element of insulating material, the core element having a longitudinally extending slot disposed in its outer wall, and a movable contact arm passing within the core element along the longitudinal slot, the contact arm making a movable pressure contact with the wound resistance wire passing over the longitudinal slot, means positioned on the wound resistance wire along the longitudinal slot remote from the core element to provide backing support for the wound resistance wire against the movable pressure contact of the movable arm comprising a hard, rigid backing bar composed of a hard insulating material chosen from the group of materials consisting of Pyrex-type glass, ceramics, lavas, aluminas, glass, dialyl-phthalate plastics and hard anodized aluminum, the backing bar being slightly narrower than the longitudinal slot to allow the backing bar to fit into the longitudinal slot and having a lower face contacting the wound resistance wire along the longitudinal slot and an upper face which cooperates with means for forcing the backing bar into the longitudinal slot to cause the wound resistance wire passing over the longitudinal slot to be depressed into the slot to follow the contour of the lower face of the backing bar whereby the wound resistance wire makes contact with the movable arm within the slot.

3. In an adjustable wire-wound system including a resistance wire wound over a hollow core element of insulating material, the core element having a longitudinally extending slot disposed in its outer wall, and a movable contact arm passing within the core element along the longitudinal slot, the contact arm making a movable pressure contact with the wound resistance wire passing over the longitudinal slot, the combination of: a hard, rigid backing bar composed of a hard insulating material chosen from the group of materials consisting of Pyrex-type glass, ceramics, lavas, aluminas, glass, dialyl-phthalate plastics and hard anodized aluminum, the bar being slightly norrower than the longitudinal slot to allow the bar to fit into the slot, and having a curved lower face contacting the wound resistance wire along the longitudinal slot and a curved upper face having a longitudinal groove extending the length of the backing bar; a conductor disposed within the longitudinal groove, the conductor being longer than the backing bar such that end portions of the conductor extend beyond the ends of the backing bar, the end portions forming terminal posts for the wound resistance wire; and a sleeve of insulating material having an inner surface which cooperates with curved upper surface of the backing bar to form a snug fit around the wound resistance wire thereby forcing the backing bar into the longitudinal slot to depress the wound resistance wire passing over the longitudinal slot to follow the contour of the curved lower face of the backing bar and make tangential contact with the movable contact arm within the core element.

4. In an adjustable wire-wound system including a resistance wire wound over a hollow core element of insulating material, the core element having a longitudinally extending slot disposed in its outer wall and a movable contact arm passing within the core element along the longitudinal slot, the contact arm making a movable pressure contact with the wound resistance wire passing over the longitudinal slot, the combination of: a hard, rigid backing bar composed of a hard insulating material chosen from the group of materials consisting of Pyrextype glass, ceramics, lavas, aluminas, glass, dialyl-phthalate plastics and hard anodized aluminum, the bar being slightly narrower than the longitudinal slot to allow the backing bar to be depressed into the longitudinal slot and having outwardly curved upper and lower faces, the curved lower face contacting the wound resistance wire along the longitudinal slot, the curved upper face having a longitudinal groove extending the length of the bar; a conductor disposed within the longitudinal groove, the conductor being longer than the backing bar such that end portions of the conductor extend beyond the ends of the backing bar, the end portions forming terminal posts for the wound resistance wire; and means cooperating with the curved upper face of the backing bar for forcing the backing bar into the longitudinal slot to depress the 7 Wound resistance wire passing over the longitudinal slot to follow the contour of the curved lower face of the backing bar thereby causing the wound resistance wire to make a tangential contact with the contact arm within the core element.

5. In an adjustable wire-wound system including a resistance wire wound over a hollow core element of insulating material, the core element having a longitudinally extending slot disposed in its outer wall and a movable contact arm passing within the core element along the longitudinal slot, the contact arm making a movable pressure contact with the wound resistance wire passing over the longitudinal slot, the combination of: a hard, rigid backing bar composed of a hard insulating material chosen from the group of materials consisting of Pyrex-type glass, ceramics, lavas, aluminas, glass, dialyl-phthalate plastics and hard anodized aluminum, the backing bar having upper and lower faces and being slightly narrower than the longitudinal slot to allow the backing bar to fit into the longitudinal slot, the lower face contacting the wound resistance wire along the longitudinal slot; and means cooperating with the upper face of the backing bar for forcing the backing bar into the longitudinal slot to depress the wound resistance wire passing over the longitudinal slot to follow the contour of the lower face of the backing bar whereby the wound resistance wire makes contact with the movable contact arm Within the core element.

6. In an adjustable wire-wound system including a resistance wire wound over a hollow core element of insulating material, the core element having a longitudinally extending slot disposed in its outer wall, and a movable contact arm passing within the core element along the longitudinal slot, the arm making a movable over the slot, the combination of: a hard, rigid backing bar composed of a hard insulating material mounted external to the wound resistance wire over the longitudinal slot, the bar having a lower surface contacting the wound resistance wire; and means for forcing the bar into the longitudinal slot to depress the wound resistance wire passing over the slot to follow the contour of the lower surface of the bar to make contact with the movable contact arm within the slot.

7. In an adjustable wire-wound system including a resistance wire wound over a hollow core element of insulating material, the core element having a longitudinally extending slot disposed in its outer wall, and a movable contact arm passing within the core element along the longitudinal slot, the arm making a movable pressure contact with the wound resistance wire passing over the slot, the combination of: a hard, rigid backing bar composed of a hard insulating material mounted external to the wound resistance wire over the longitudinal slot, the backing bar having a lower surface contacting the Wound resistance wire and being narrower than the longitudinal slot; and means for forcing the bar into the l0ngitudinal slot to depress the wound resistance wire passing over the slot to make contact with the movable contact arm within the slot.

8. A backing bar for an electrical Wire-Wound system composed of a hard insulating material chosen from a group of materials consisting of Pyrex-type glass, ceramics, lavas, aluminas, glass, and dialyl-phthalate plastics, the backing bar having outwardly curved opposing faces and a longitudinally extending groove in a one of the faces for receiving a conductor.

References (Iited in the file of this patent UNITED STATES PATENTS 2,892,171 Semple June 23, 1959 

1. IN AN ADJUSTABLE WIRE-WOUND SYSTEM INCLUDING A RESISTANCE WIRE WOUND OVER A HOLLOW CORE ELEMENT OF INSULATING MATERIAL, THE CORE ELEMENT HAVING A LONGITUDINALLY EXTENDING SLOT DISPOSED IN ITS OUTER WALL, AND A MOVABLE CONTACT ARM PASSING WITHIN THE CORE ELEMENT ALONG THE LONGITUDINAL SLOT, THE CONTACT ARM MAKING A MOVABLE PRESSURE CONTACT WITH THE WOUND RESISTANCE WIRE PASSING OVER THE LONGITUDINAL SLOT, MEANS POSITIONED ON THE WOUND RESISTANCE WIRE ALONG THE LONGITUDINAL SLOT REMOTE FROM THE MOVABLE CONTACT ARM TO PROVIDE BACKING SUPPORT FOR THE WOUND RESISTANCE WIRE AGAINST THE MOVABLE PRESSURE CONTACT OF THE MOVABLE ARM COMPRISING A HARD, RIGID BACKING BAR COMPOSED OF A HARD INSULATING MATERIAL CHOSEN FROM THE GROUP OF MATERIALS CONSISTING OF PYREX-TYPE GLASS, CERAMICS, LAVAS, ALUMINAS, GLASS, DIALYL-PHTHALATE PLASTICS AND HARD ANODIZED ALUMINUM, THE BAR BEING SLIGHTLY NARROWER THAN THE LONGITUDINAL SLOT TO ALLOW THE BACKING BAR TO FIT INTO THE SLOT AND HAVING AN OUTWARDLY CURVED LOWER FACE CONTACTING THE WOUND RESISTANCE WIRE ALONG THE LONGITUDINAL SLOT, AND AN UPPER FACE HAVING A LONGITUDINALLY EXTENDING GROOVE FOR RECEIVING A CONDUCTOR, THE UPPER FACE BEING OUTWARDLY CURVED TO COOPERATE WITH MEANS FOR FORCING THE BAR INTO THE SLOT TO DEPRESS THE WOUND RESISTANCE WIRE PASSING OVER THE LONGITUDINAL SLOT INTO THE SLOT TO FOLLOW THE CONTOUR OF THE CURVED LOWER FACE OF THE BACKING BAR WHEREBY THE WOUND RESISTANCE WIRE MAKES TANGENTIAL CONTACT WITH THE MOVABLE ARM WITHIN THE SLOT. 